1. Field of the invention
The invention relates to a permanent magnet type stepping motor, and more particularly to an improvement of the stator subassembly thereof.
2. Related art
FIG. 1 shows a construction of an ordinary permanent magnet type stepping motor 1. The stepping motor 1 includes a cylindrical rotor 2 and an annular stator 3 that surrounds the outer circumference of the rotor 2. The rotor 2 is constructed of a permanent magnet that is magnetized alternately into S and N poles along the outer circumference, and is rotatably supported by a central rotor shaft 4 with respect to a not shown motor case.
Further, the stator 3 is assembled by two stator subassemblies 5, 6. These stator subassemblies 5, 6 are integrally molded with a resin portion 15 while arranged in the axial direction of the rotor shaft 4. The stator subassemblies 5, 6 are formed of two annular stator cores 7, 8 and stator cores 9, 10, respectively.
These stator cores 7, 8, 9, 10 have comblike polar teeth 11, 12, 13, 14 in a central part thereof so as to correspond to the outer circumferential surface of the rotor 2, and are combined so as to alternate the polar teeth 11, 12, 13, 14 in terms of position so that the phases are shifted in a direction of rotation.
The resin portion 15 integrates the two stator subassemblies 5, 6, i.e., four stator cores 7, 8, 9, 10, and forms bobbins 16, 17 by covering the outer side surface of these stator subassemblies with a resin, i.e., an insulator to provide therein a space for accommodating coils 18, 19. At the time of molding the resin portion 15, the resin is charged from one or two or more gate portions 20 formed on the side surface of the resin portion 15. Holes 21 are formed at an appropriate interval on flange portions of the stator cores 7, 8, 9, 10. As the resin is introduced into these holes, the combining of these portions made of different materials is reinforced. It may be noted that the polar teeth 11, 12, 13, 14, being located on the same surface as the inner circumferential surface of the resin portion 15, are exposed only from the inner circumferential surface and therefore confront the outer circumferential surface of the rotor (permanent magnet).
FIG. 2 shows how the stator subassemblies 5, 6 (stator cores 7, 8, 9, 10) are combined. As shown in FIG. 2, the stator cores 8, 9 in the middle which are disposed back to back are located out of phase with each other, so that the gap between the polar teeth 12, 13 thereof is small. As a result, when the resin is charged into a cavity between an inner mold 22 and an outer mold 23 and insert-molded, the resin does not flow smoothly at such small-gapped portion. This causes the polar teeth 11, 12, 13, 14, particularly, the polar teeth 11, 13 of the stator cores in the middle to be bent outward by the resin charging pressure, which in turn causes the resin to overhang the inner circumferential surface of the polar teeth 11, 12, 13, 14. As a result, not only the magnetic resistance is increased, but also the polar teeth 11, 12, 13, 14 are fallen outward to reduce the wall thickness of the bobbins 16, 17 at the region where the polar teeth are fallen, which imposes the problem of insulation. This shortcoming is noticeably addressed when the number of polar teeth is increased.